1. Field of the Invention
This invention relates to a new process for preparing polyanhydrides.
2. Background Information
Various polyanhydrides are known in the art as described, for example, in the references discussed in the Description of Related Art given hereinafter. Some of these, utilizing their property of hydrolytic degradation, have been. used for the controlled release of agents such as drugs, e.g., poly[bis(p-carboxyphenoxy)alkane anyhydrides], copolyanhydrides of bis(p-carboxyphenoxy)alkanes with sebacic acid, poly(terephthalic acid anhydride) and copolyanhydrides of terephthalic and sebacic acids. One method of preparing such polyanhydrides is melt condensation, wherein a prepolymer is first formed by reacting a dicarboxylic acid with acetic anhydride, and the prepolymer is separated from the reaction mixture, washed, and melt condensed at an elevated temperature to form the polyanhydride. A disadvantage of this method is that it requires two steps with a necessary separation and purification of the prepolymer produced by the first step. Moreover, the reversible thermal depolymerization may limit the highest molecular weight obtainable. A still further disadvantage is that in the case of copolyanhydrides, random polymers are produced, which are subject to less controlled rates of hydrolytic degradation, rather than regularly alternating polymers which tend to degrade hydrolytically in a more controlled manner.
Another method of preparing polyanhydrides is by the dehydrochlorination of a free dicarboxylid acid and a diacid chloride (Schotten-Baumann condensation). However, this method always requires at least one solvent (two for interfacial polymerization), and yields corrosive hydrogen chloride gas as a by-product.
A third method of preparing polyanhydrides is dehydrative coupling of the free dicarboxylic acid using a coupling agent such as an organophosphorus compound. However, it is difficult to obtain polymers of relatively high molecular weight using this method.